Self-study
Support In experiments in which certain kinds of bacteria were placed in a generous supply of nutrients, the populations of bacteria grew rapidly, and genetic mutations occurred at random in the populations. █████ ███████████ ████ ████ ███ ███████ ████████ ██ ███████
Objective: Identify a Sufficient Assumption
The author concludes that all genetic mutation is random. How do we know? Because in an experiment, certain bacteria placed in a nutrient-rich environment multiplied rapidly and developed random genetic mutations.
To reach the conclusion, the author makes an extreme generalization from limited evidence. An experiment that's just about certain types of bacteria in a specific environment doesn't actually support a claim about all genetic mutations, in all species, in all circumstances. So we need to bridge the gap between the support and the conclusion to allow the conclusion to be properly drawn. The bridge needs to allow the experimental result to actually support such a broad conclusion—it should tell us that if genetic mutations are ever random, then all genetic mutations are random.
18.
Which one of the following, ██ █████ ███████ ███ ██████████ ██ ██ ████████ ██████
a
Either all genetic █████████ ███ ██████ ██ ████ ███ ███████
(A) presents a different phrasing of our predicted answer, and therefore guarantees the conclusion. (A) presents a choice between randomness and non-randomness, which is triggered by the evidence some genetic mutations are random to yield a conclusion that all genetic mutations must be random.
b
The bacteria tested ██ ███ ███████████ ████ ██ █████████ ██████ ██████
The bacteria in the experiment being common doesn't make the argument hold water. The argument's flaw of over-generalization still exists regardless of whether the bacteria used were common or rare. Being common doesn't make these bacteria representative of all life-forms.
c
If all genetic █████████ ██ ████████ ███ ███████ ████ ███ ███████ █████████ ██ █████ █████ ████ ████ ███ ██████ █████
(C) is tempting, but when reading closely, the argument doesn't trigger (C) because the sufficient condition presented by (C) isn't met. The argument doesn't actually establish that all genetic mutations in bacteria are random, only that some genetic mutations in bacteria are random. Because (C) isn't triggered, it can't bridge to the conclusion that all genetic mutations are random.
Fundamentally, the argument's flaw of over-generalization would still be present even if the conclusion were limited to bacteria. The argument only establishes that genetic mutations in bacteria are sometimes random—for all we know, other bacteria in other conditions would have non-random mutations. We can't make the leap to assume that all genetic mutations in bacteria are random, because the argument's support doesn't even get us that far.
d
The kind of ███████████ ██ █████ ███████ ████████ █████ █████ ███ ██ ██████ ██ ███ ███ ███████ ████████ ███████
(D) provides one piece of the puzzle, but it doesn't go far enough to guarantee the conclusion of the argument. Even if the environment doesn't make a difference, what about the type of organism? The argument doesn't consider any examples except bacteria, so that's another variable the correct answer needs to account for. Compared to a more comprehensive assumption like (A), (D) still leaves gaps in the argument.
e
The nutrients used ████ ███ ████ ██ █████ ████ ███████ ███ ████████ ██ ███████
The author is claiming that all genetic mutations are always random, so distinguishing between nature and artificial environments doesn't make a difference. If (E) were true, but there were also artificial nutrients that caused non-random mutations, that would still disprove the argument. We can see from that possibility that (E) doesn't do enough given the absolute nature of the conclusion.